The present invention relates to a deflection control ink jet recording apparatus which ejects ink under pressure from a nozzle, applies vibration to the ejected ink to form ink droplets regularly, develops selectively a charging electric field according to an image signal when each ink droplet shapes itself, charges the ink droplets by the electric field, and deflects the charged ink droplet by a deflecting electric field. More particularly, the present invention is concerned with a device associated with a deflection control ink jet recording apparatus of the type having a linear arrangement of numerous ink ejection holes in order to determine proper levels of charging voltage.
Known ink jet recording apparatus of the type described may be classified generally into a two-value deflection control apparatus, a multi-value deflection control apparatus and a combined apparatus of the two mentioned. In the first or two-value apparatus, ink droplets for printing data are charged (or charged to a high level) while those which are not used for printing are left non-charged (or charged to a low level or to the opposite polarity) so that the recording droplets may be deflected to a large extent by a deflecting electric field to impinge on a recording sheet and the non-recording droplets may be captured by a gutter. Conversely, the non-recording ink may be deflected to a large extent to be captured by a gutter. In this type of apparatus, one nozzle is used for one picture element during the recording operation. In the second or multi-value apparatus, one nozzle is used for three or more picture elements (e.g. 5 mm and 40 dots, assuming 8 dots/mm) and recording droplets of ink are charged to three or more levels (e.g. 40 levels) to be deflected along three or more paths (e.g. 40 paths). In the third or combined apparatus, recording ink droplets are charged in the same way as in the multi-value process. However, this last-mentioned apparatus first deflects recording charged droplets using a deflecting electric field extending in the Y-axis direction so as to cause them to miss a gutter and then deflects them using another electric field in the X-axis direction in accordance with their charging levels, thereby printing out data in the X direction on a recording sheet with positional variations.
Meanwhile, ink to be ejected from a nozzle may be vibrated by any of three known systems: one which imparts pressure oscillation to the ink proper, one which imparts vibration in an intended direction of ink ejection to a nozzle plate having at least one ink ejection hole, and one which applies vibration bodily to an ink ejection head in an intended direction of ink ejection. The first system permits the use of a single nozzle plate having one ejection hole which is bonded to the leading end of a cylindrical electrostrictive vibrator, the other end of which is communicated with a pressurized ink supply box. It also permits the use of a nozzle plate having numerous ink ejection holes which is bonded to the front wall of a pressurized ink supply box in such a manner as to cover a slit provided in said wall of the ink supply box. One or more flat electrostrictive vibrators are mounted on one side wall of the box to impart vibrating pressure to ink inside the box. The second system employs a multi-apertured nozzle plate rigidly mounted to a pressurized ink supply box through an elastic member which is caused to vibrate by an electrostrictive vibrator. The third system drives a head bodily for oscillation by means of a motor, a solenoid device, an electrostrictive vibrator or the like.
A deflection control ink jet recording apparatus of any of the systems stated places a recording sheet at a relatively large spacing from its nozzle plate. For this reason, ink is pressurized to a level high enough for a droplet of ink from the nozzle to reach the recording sheet stably along a predetermined path despite its passage through the charging and deflecting electrodes. In order that ink droplets of a given diameter may appear regularly and follow their predetermined paths accurately, there must be stabilized and exactly controlled a variety of factors including the viscosity and pressure of ink, vibrating pressure, amount of charge and intensity of deflecting electric field. It is impossible, however, to hold all of such quantities under fully ideal conditions. This particularly results in misalignment of actual deflection paths from reference deflection paths in the case of the multi-value deflection control which charges ink ejected from a single ejection hole to several different levels and drives them to different positions on a recording sheet.
Where in multi-value deflection ink droplets are charged to 40 levels, they are expected to move along 40 predetermined paths. In practice, however, the individual ink droplets tend to become offset from the predetermined paths due to the dense droplet distribution in the paths and unwanted mutual influence of the droplets caused by the very short distance between adjacent droplets and charges of the same polarity on the droplets. Another factor causative of such dislocation is that a charge on a droplet is disturbed by a charge on the immediately preceding droplet depending on the amount of charge on the latter.
It has been proposed in U.S. Pat. No. 3,787,882 to control the ink pressure by detecting at least one of the ink pressure, temperature, flying velocity and amount of deflection. However, ink pressure control without any assistance cannot afford sufficient stability, accuracy or response for the control of deflection positions of ink droplets, that is, the control of printing positions because the recording density is as high as 4 dots per mm, 8 dots per mm or the like and thus requires very delicate deflection control. For this reason, it is usually preferred to control the amounts of deflection through adjustment of voltage levels for charging ink droplets. Charging voltage levels can be adjusted delicately and quickly over such a wide range. Even if the amounts of deflecton vary as a result of changes in at least one of the ink viscosity, ink pressure, power source voltage, driving characteristic and ejection conditions of ink at ink ejection ports, the adjustment of charging voltage levels will succeed in confining the amounts of deflection to predetermined values.
A prior art deflection amount control system employing adjustment of charging voltage levels is designated to determine a required charge voltage by computing a charge voltage for directing a charged ink droplet to a predetermined deflection position or its amount of variation from the correlation between a charge voltage and an offset of a droplet charged by said charge voltage from a predetermined deflection position, and set the thus determined charge voltage as an adequate charge voltage. Another known system of the type described detects an actual deflection position of a charged ink droplet and varies the charging voltage level step by step to bring the actual deflection position closer to a predetermined reference deflection position. In any of these known control systems, a charge voltage corresponding to a one step of change in the charge voltage level is fixed and the accuracy of deflection amount setting operation is dependent on the charge voltage corresponding to the one step of change. Thus, the charge voltage corresponding to one step of change has a relatively small value. It follows that a substantial offset of the actual deflection position of a charged droplet from the reference position must be corrected by repeated changes of charge voltage level which consumes a relatively long period of time.